Effect of contact force on diode characteristics of Rh/epitaxial graphene/n-SiC

Japanese Journal of Applied Physics, Part 1 (Special Issues)


Twist angle dependence of graphene-stacked junction characteristics


Vertically stacked graphene tunnel junction with structured water barrier

Japanese Journal of Applied Physics, Part 1 (Regular Papers & Short Notes)


Ion sensitivity of large-area epitaxial graphene film on SiC substrate


Carrier doping effect of humidity for single-crystal graphene on SiC


Fabrication of hydrophilic graphene film by molecular functionalization

Physica Status Solidi (B) Basic Solid State Physics : PSS


Epitaxial graphene on SiC formed by the surface structure control technique

The thermal decomposition of silicon carbide (SiC) is a promising method for producing wafer-scale single-crystal graphene. The optimal growth condition for high-mobility epitaxial graphene fabricated by infrared rapid thermal annealing is discussed in this paper. The surface structures, such as step terrace and graphene coverage structures, on a non-off-axis SiC(0001) substrate were well controlled by varying the annealing time in a range below 10 min. The mobility of graphene grown at 1620 °C for 5 min in 100 Torr Ar ambient had a maximum value of 2089 cm2 V%1 s%1. We found that the causes of the mobility reduction were low graphene coverage, high sheet carrier density, and nonuniformity of the step structure.


Effects of UV light intensity on electrochemical wet etching of SiC for the fabrication of suspended graphene

We report on the effects of UV light intensity on the photo assisted electrochemical wet etching of SiC(0001) underneath an epitaxially grown graphene for the fabrication of suspended structures. The maximum etching rate of SiC(0001) was 2.5 m/h under UV light irradiation in 1 wt % KOH at a constant current of 0.5 mA/cm2. The successful formation of suspended structures depended on the etching rate of SiC. In the Raman spectra of the suspended structures, we did not observe a significant increase in the intensity of the D peak, which originates from defects in graphene sheets. This is most likely explained by the high quality of the single-crystalline graphene epitaxially grown on SiC.


Surface-Enhanced Raman Scattering of Graphene on SiC by Gold Nanoparticles

金微粒子によるSiC上グラフェンの表面増強ラマン散乱


Stability and reactivity of steps in the initial stage of graphene growth on the SiC(0001) surface

The stability and reactivity of steps in the initial stage of the graphene growth by sublimating the SiC(0001) surface is theoretically studied by the first-principles calculation. The steps are not necessarily unstable and reactive during the formation of the zeroth graphene layer. Temperature, Si pressure, and C coverage affect the stability and reactivity of the step. The growth mode shows a phase transition. The step is unstable and reactive after the zeroth graphene layer is formed. These findings are tightly related to the graphene formation mechanism on this surface. They are discussed with experimental results and suggest a way to control the quality of graphene.

Physical Review B, Condensed Matter and Materials Physics


Microscopic Raman mapping of epitaxial graphene on 4H-SiC (0001)

We propose a quality control method for wafer-scale epitaxial graphene grown on SiC substrates. The peak position of Raman spectra of epitaxial graphene is an excellent indicator of film quality and reveals irregularities, such as graphene thickness inhomogeneity and SiC substrate defects. A comparison of microscopic Raman maps and scanning probe microscopy images of the same position of the sample revealed that wave numbers of Raman peaks (G and 2D band peaks) were strongly correlated with the strain in the graphene film. The increase in number of graphene layers (2 to 3--4 layers) induced phonon softening ({im}6 cm-1) and broadening ({im}6 cm-1) of the 2D band peak. Significant phonon softening and abnormal broadening of the Raman peaks were observed at residual scratches on the SiC substrate. The quantitative layer number distribution of graphene on SiC is successfully estimated from the wave number distribution of the 2D band peak.


Atomic structure of epitaxial graphene islands on SiC(0001) surfaces and their magnetoelectric effects


Significance of the interface regarding magnetic properties of manganese nanosilicide in silicon


Carrier transport mechanism in graphene on SiC(0001)


Theoretical Study on Epitaxial Graphene Growth by Si Sublimation from SiC(0001) Surface

The growth of epitaxial graphene on the SiC(0001) surface is theoretically studied by assuming silicon (Si) sublimation from the surface. Our results indicate that a new graphene sheet grows from the interface between the old graphene sheets and SiC substrate, as found in our previous study on graphene growth by carbon (C) deposition. Graphene growth requires overcoming rather lower energy barriers until 0-monolayer graphene (buffer layer) is formed. Further growth toward formation of 1-monolayer graphene requires overcoming energy barriers higher by about 0.7 eV, which indicates that the growth preferably stops once when 0-monolayer graphene is formed. Compared with the C deposition case, the growth requires overcoming the energy barrier higher by about 0.7 eV, which indicates that the graphene growth is more difficult. In addition, the nonuniform growth of surface C aggregates is thought to degrade the quality of the grown graphene. The C-rich condition is therefore important for obtaining high-quality graphene. The experimental graphene growth is considered to proceed similarly to the C deposition case rather than the Si sublimation case.

Japanese Journal of Applied Physics, Part 1 (Regular Papers & Short Notes)


Theoretical Study on Magnetoelectric and Thermoelectric Properties for Graphene Devices

Two of our recent theoretical efforts on elucidating the functions of graphene are reported. A first-principles calculation of the growth process of graphene islands on SiC(0001) shows that an embedded structure is energetically preferable. Island with this embedded structure do not have any broken dangling bonds at their edges. Their electronic states clearly show that they surely act as islands. Islands with zigzag edges have edge-localized states, which causes magnetoelectric effects. Graphene is also expected as a highly efficient material for thermoelectric elements according to a theoretical study. If the carrier scattering sources are adequately suppressed, the thermoelectric figure of merit greatly exceeds 1 at temperatures higher than 300 K with the Fermi energy fixed around the Dirac point. Since graphene is cheaper, resource abundant, more harmless, higher in melting temperature, and much lighter in density, than the present typical material, BiTe/Sb, many new applications could be considered.


X-ray diffraction profiles of Si nanowires with trapezoidal cross-sections


Observation of band gap in epitaxial bilayer graphene field effect transistors

Bilayer graphene was grown on the Si-face of SiC by thermal decomposition. Its electronic properties were investigated in top-gate Hall bar devices. By controlling the carrier density using gate voltage, we were able to access the charge neutrality point. The conductance at the charge neutrality point showed a strong temperature dependence, and its temperature dependence was well fitted with thermal activation and variable-range hopping mechanisms. The electrical detection of a band gap opening in bilayer graphene grown on SiC is a promising step toward the realization of graphene-based electronics using epitaxial graphene.


Separately contacted monocrystalline silicon double-layer structure with an amorphous silicon dioxide barrier made by wafer bonding


Epitaxial few-layer graphene: toward single crystal growth


Direct Actuation of GaAs Membrane Resonator by Scanning Probe


Local Conductance Measurement of Double-layer Graphene on SiC Substrate

The microscopic structural and electrical properties of few-layer graphene grown on an SiC substrate were characterized by low-energy electron microscopy, transmission electron microscopy and scanning probe microscopy measurements of local conductance. The double-layer graphene sheet was confirmed to be continuous across the atomic steps on the buried SiC substrate surface, and the measured local conductance was clearly modified in the vicinity of the steps. The conductance decreased (slightly increased) at the lower (upper) side of the steps, suggesting deformation-induced strain is the origin of the conductance modification. From the contact force dependence of the conductance images, the effective contact areas for both nanogap-probe and point-probe measurements were estimated.


Stacking domains of epitaxial few-layer graphene on SiC(0001)

Physical Review B, Condensed Matter and Materials Physics


Evaluation of Thermal Mechanical Vibration Amplitude and Mechanical Properties of Carbon Nanopillars Using Scanning Electron Microscopy

We describe a method for evaluating thermal mechanical vibration amplitude by means of analysis of scanning electron microscopy images. The samples used were carbon nanopillars of different heights grown by focused-ion-beam-induced chemical vapor deposition. The secondary electron yield profile of carbon nanopillars excited by thermal noise is modeled, and vibration amplitude is determined by fitting the modeled profile to the experimental profile. The Young's modulus of carbon nanopillars is deduced from the determined amplitude. Furthermore, the density of carbon nanopillars is estimated from the deduced Young's modulus and the measured resonant frequency. The obtained Young's moduli and densities range from 51 to 78 GPa and from 2500 to 3500 kg/m3, respectively.


Direct Actuation of GaAs Membrane with the Microprobe of Scanning Probe Microscopy

A method for evaluating the dynamic characteristics of micro- and nanoresonators with high spatial resolution is proposed. The mechanical resonance of circular micromembrane resonators is directly induced by voltage applied from the microprobe of a scanning probe microscopy (SPM) system. The vibration amplitude is simultaneously detected as height information by SPM. Experimentally, the resonant properties of fundamental and higher-order modes of 200-nm-thick GaAs micromembranes were measured. The frequency of the highest mode is 3.4 MHz and its resonant amplitude is about 1 nm. The resonant amplitude increases with increasing actuation voltage in a linear manner at voltages below 180 mV. Large actuation voltage induces nonlinear vibration with the spring soften effect, which originates from the strong attractive force induced by the electronic field between the probe and membrane. The high tapping force, which is repulsive, induces another type of nonlinear vibration caused by the spring harden effect. The simultaneous actuation and detection for mechanical resonators based on SPM technology reveals the characteristics of the mechanical interaction force between the micromembrane and microprobe.

Japanese Journal of Applied Physics, Part 1 (Regular Papers & Short Notes)


Heterogeneous Integration on Graphene Substrate --- 特集/Heterogeneous Integration

グラフェン基板上への異種機能集積化について --- 特集/Heterogeneous Integration

Journal of Japan Institute of Electronics Packaging

エレクトロニクス実装学会誌


Study on Graphene Composite Properties for New Functional Devices

グラフェン複合物性の機能デバイス化技術の研究

This study describes formation and evaluation techniques of graphene on SiC for new functional devices using composite properties. A new layer number determination technique for graphene on SiC was established using microscopic Raman spectroscopy. Growth mechanism of graphene was revealed by detailed image analysis of scanning probe microscopy (SPM). Highly uniform single-layer single-crystal graphene was successfully grown on SiC substrate of 10 mm-sq size. New methods for mechanical and electrical properties of graphene were also developed. Friction force of graphene on SiC was evaluated using friction force microscopy. Contact conductance properties were measured using conductive nanoprobes on SPM.

Bulletin of Institute of Technology and Science, The University of Tokushima

徳島大学大学院ソシオテクノサイエンス研究部研究報告


Metrology of microscopic properties of graphene on SiC --- [Invited Paper]

Graphene has recently attracted a lot of research interest because of its superior electric properties. Thermally grown epitaxial graphene on SiC substrate is promising for future electronic devices because of its compatibility with existing wafer-scale manufacturing. In this paper, microscopic metrological methods for graphene on SiC will be discussed. A layer number determination method using low-energy electron microscopy (LEEM) enables us to control the layer number and morphology of few-layer graphene. Local conductance measurements using an integrated nanogap probe based on scanning probe microscopy reveal the electrical properties of graphene nanoislands and double-layer graphene sheets on SiC.


Planar cold cathode based on a multilayer-graphene/SiO2/Si heterodevice


Resistivity anisotropy measured using four probes in epitaxial graphene on silicon carbide


Graphene-Based Nano-Electro-Mechanical Switch with High On/Off Ratio

Locally defined nanomembrane structures can be produced in graphene films on a SiC substrate with atomic steps. The contact conductance between graphene and a metal-coated nanoprobe in scanning probe microscopy can be drastically reduced by inducing local buckling of the membranes. Repeatable current switching with high reproducibility can be realized. The on/off ratio can be varied from about 105 to below 10 by changing the contact force. At a low contact force, the contact conductance changes from 10μS (``ON'' state) to 100pS (``OFF'' state). This novel device structure could represent a new path to electrical switching at the nanoscale.


Atomic Structure and Physical Properties of Epitaxial Graphene Islands Embedded in SiC(0001) Surfaces

The atomic structures of graphene islands on SiC(0001) surfaces are studied theoretically together with their growth mechanism. Two types of embedded graphene island structure are proposed. It is shown that these structures actually act as the graphene island electronically, and that those with zigzag edges have the magnetoelectric effect.


Half-Integer Quantum Hall Effect in Gate-Controlled Epitaxial Graphene Devices

High-quality monolayer graphene was grown on the Si face of SiC by thermal decomposition, and its electrical properties were investigated in top- gated devices. At 2 K, the carrier mobility of the graphene exceeded 10,000 cm2 V 1 s 1 and the half-integer quantum Hall effect was observed. The quantum Hall states were even observed at various carrier densities when top-gate bias was applied. These findings suggest high-quality epitaxial graphene possesses the unique nature of monolayer graphene and is robust against device fabrication, which holds potential for graphene-based electronics applications.


Contact Conductance Measurement of Locally Suspended Graphene on SiC


Theoretical Study of Epitaxial Graphene Growth on SiC(0001) Surfaces

The epitaxial graphene growth mechanism on SiC(0001) surfaces is studied based on the energetics via the first-principles calculation. It is found that a Si terminated surface is important for the epitaxial growth of thin flat graphene sheets. This surface encourages the growth of graphene sheets because the surface Si atoms act as catalyst. Si desorbed sites trap excess C atoms, and form graphene islands. It is also found that the new graphene sheet prefers to grow just on the Si terminated surface of the SiC substrate even if the surface is covered with other graphene sheets.